1. Field of the Invention
The present invention relates to a transmission rate changing technology. In particular, the invention relates to a transmission rate changing method for changing a transmission rate during communication, and a base station apparatus using the same.
2. Description of the Related Art
In wireless communication systems, transmission links vary in environment with time. Among technologies for increasing the transmission rate in such transmission link environment is adaptive modulation scheme. The adaptive modulation scheme controls its modulation mode in accordance with the transmission link environment. For example, if the transmission link environment is determined to be unfavorable, a modulation mode of high reliability is used for data transmission. If the transmission link environment is determined to be favorable, on the other hand, a modulation mode of high information volume is used for data transmission. When a base station apparatus and a terminal apparatus included in a wireless communication system communicate by TDD (Time Division Duplex) scheme, slots are typically transmitted for an up link and a down link alternately in regular order.
Each of the slots contains a preamble to be used for estimating the transmission link environment, modulation mode information indicating the modulation mode used for transmission, and information symbols. When the transmission links of TDD scheme are reversible, the base station apparatus receives a slot, detects the C/N (Carrier to Noise Ratio) of the transmission links, delay spread, and from received level information including a received baseband signal and RSSI (Received Signal Strength Indicator), and estimates the transmission link environment at the next timing of transmission. Besides, based on this estimation result, the base station apparatus selects a modulation mode to be used for transmission (See reference (1) in the following Related Art List, for instance).